1. Technical Field
The present disclosure relates generally to chemical pulping and particularly to recovery boilers and dissolving tanks used in the pulp and paper industry.
2. Related Art
Chemical pulping converts lignocellulosic biomass to pulp fibers of various lengths. In the pulp and paper industry, the lignocellulosic biomass often comprises wood chips; but lignocellulosic material may include other plant-based biomass in which the protein lignin is closely associated with cellulosic sugar molecules. With processing, operators can isolate cellulosic pulp fibers for use in a variety of commercial applications, including paper manufacturing.
When wood is the primary lignocellulosic material for example, production may begin with a log. A debarker removes the bark from (or “debarks”) logs, and a chipper comminutes the logs into small chips. Depending on the particular process and application, operators may pretreat these chips with steam and chemicals to expand pores in the lignocellulosic biomass, or operators may send dried chips directly into a chemical digester. Continuous chemical digesters are generally cylindrical and may be several stories high.
In the digester, operators typically introduce white liquor and steam into the digester's upper section. In the Kraft process, the “white liquor” often consists of a sodium hydroxide and sodium sulfide solution. Over the course of several hours, the steamed biomass moves through the digester as white liquor dissolves the lignin. Lignin is a protein that binds the cellulose and hemicellulose in the biomass together. Removal of lignin permits operators to isolate fibers comprising mainly cellulose and hemicellulose. As the lignin and other ancillary biomass compounds dissolve into the liquor, the liquor darkens and becomes “black liquor”.
After the black liquor solution exits the digester, equipment isolates the cellulosic pulp fibers from the remaining black liquor. Whereas white liquor contains sodium hydroxide and sodium sulfide, the black liquor contains sodium carbonate and sodium sulfate respectively. Sodium carbonate and sodium sulfate are the products of the white liquor's chemical reaction with the lignin and other compounds in the digester. The products, sodium carbonate and sodium sulfate, are generally less useful for digesting lignin.
While sodium hydroxide and sodium sulfide are generally inexpensive chemicals, purchasing new solutions of sodium hydroxide and sodium sulfide for every new batch of lignocellulosic biomass is generally cost prohibitive. For this reason, many chemical pulp mills use pyrolytic chemical recovery systems to convert at least a portion of the sodium carbonate and sodium sulfate back into useful sodium hydroxide and sodium sulfide.
New black liquor from a chemical digester is generally dilute and non-combustible. Therefore, to prepare black liquor for pyrolysis, operators generally funnel the black liquor through flash tanks or other evaporation steps to increase the amount of solid particles concentrated in the black liquor. Operators then heat the concentrated black liquor before injecting the concentrated black liquor through spray nozzles into a chemical recovery boiler. The spray nozzles create coarse droplets. The recovery boiler evaporates the remaining water from the droplets and the solid compounds in the black liquor undergo partial pyrolysis. The inorganic compounds that remain fall to the bottom the furnace and accumulate in a char bed. Some of the carbon and carbon monoxide in the char bed can act as catalysts to convert sodium sulfate into sodium sulfide, which can then be collected from flue gas near the top of the furnace.
The remaining inorganic compounds in the char bed eventually melt and flow as a smelt through one or more smelt spouts at the bottom of the recovery boiler. Coolant, usually water, may cool the smelt spouts. Coolant tubes may either be integrated into the spout itself, or an ancillary cooling system. The ancillary cooling system is often called a “water jacket” and may surround the outside of the spout. The smelt flowing from the spout falls into a dissolving tank and contacts water or weak white liquor to produce soda lye. The resulting soda lye solution is commonly known as “green liquor.”
In a sulfate chemical process, such as the Kraft process, the main component of the green liquor is typically sodium sulfide and sodium carbonate. However, different chemical processes produce green liquor with different inorganic compounds. Operators typically collect the green liquor and transport the green liquor to a causticizing plant to further isolate and concentrate the sodium sulfide and sodium carbonate and thereby reproduce white liquor.
As the smelt contacts the green liquor in the dissolving tank, the smelt explodes and emits a series of audible noises. This is generally known as “banging” by those in the industry. The smelt flowing from the spout is typically between 750 degrees Celsius (° C.) to 820° C., while the average temperature of the green liquor is about 70° C. to 100° C. Moreover, the smelt generally contains reactive alkali metals such as sodium, which reacts explosively with water. Without being bounded by theory, the large temperature differential may increase the reactivity of the smelt and green liquor and thereby cause or contribute to banging. If left unregulated, a sudden influx of smelt may blow up the dissolving tank and recovery boiler, which poses grave risks to nearby operating personnel.
To manage banging, conventional dissolving tanks generally disrupt the smelt as the smelt falls from the spout. Disruptors may be one or more shatter jets, which blast the falling smelt with steam or other fluid at high pressure to create smelt droplets. These droplets have a smaller volume than the overall flow of smelt and therefore, the explosions are generally less intense than they would be if the smelt contacted the green liquor as a continuous, uninterrupted, undisrupted flow. Typically, the end of the smelt spout is elevated above the level of green liquor and these shatter jets disrupt falling smelt as the smelt falls from the spout end.
Occasionally, smelt may cool prematurely in the recovery boiler or spout and decrease or eliminate the smelt flow rate. In this antediluvian state, liquid smelt tends to accumulate behind the obstruction. If the obstruction becomes dislodged, the sudden smelt influx may overwhelm the shatter jet's ability to disrupt the smelt into sufficiently small droplets. Moreover, if the deluge is particularly substantial, the smelt may flow over the sides of the spout and bypass the shatter jets entirely. In other scenarios, a shatter jet may fail. In these situations, the increased volume of smelt contacting the green liquor drastically increases the banging's explosive intensity and risk of explosion.
In many mills, operators commonly move in and amongst the processing equipment to monitor process conditions and output. An explosion in the dissolving tank or recovery boiler poses a serious safety risk to personnel in the immediate vicinity, and the resulting fire poses a serious risk to personnel in the rest of the mill. Such explosions also cause an unregulated amount of pollutants to enter the air and groundwater and predicate significant production loss. Explosions of this scale can inactivate a mill for weeks to months.